My wife's mother has lived with us since October 2013. My son cared for her for every hour my wife worked. Can I pay my son and write off his pay as a tax deduction?
If the need for care is substantiated and ordered by a physician, yes. You pay him an agreed wage, from MIL's funds, and you do the proper payroll tax withholding and issue a W-2 at the end of the year. (If you are in the US) You do not get the deduction, but the patient MIL does.
No... You could bring your MIL to a Elder Law Attorney and have him write up a care agreement in which your MIL pays both your wife and son. If you pay him from your own account that's not tax deductible..